I typed the following in a terminal
$ musique The program 'musique' is currently not installed. You can install it by typing: sudo apt-get install musique
So Ubuntu knows that "musique" is a program, although it's currently not installed on the machine. However, if I type
$ musiquez No command 'musiquez' found, did you mean: Command 'musique' from package 'musique' (universe)
it knows that there is no such program called "musiquez".
My question is, how does Ubuntu know this? Where is this information stored about what applications can be installed on Ubuntu?